Catalina, like Sierra, High Sierra and Mojave before it, references fonts that are not installed by default but are available for later download. The fonts are shown dimmed in Font Book, from which they can be downloaded. Their list is provided at Fonts included with macOS Catalina – Apple Support. Once downloaded, the fonts are installed in subfolders of /System/Library/AssetsV2/com_apple_MobileAsset_Font6, and a list of the available fonts may be found there in a file com_apple_MobileAsset_Font6.xml.
The problem is that I installed all the available fonts with a public beta of Catalina. Several fonts have been added afterwards in the release version, like Al Firat, Al Khalil, Book Antiqua, Century Schoolbook, Garamond and many others. All these fonts are listed indeed in com_apple_MobileAsset_Font6.xml. But Font Book doesn't show them (dimmed or else), offering no possibility to download them. It only shows the fonts that were there at the time of the public beta.
I've tried everything I could think of, to force Font Book to rebuild the list of available fonts: run "sudo atsutil databases -remove" and "atsutil databases -removeUser" in Terminal, erase ~/Library/Preferences/com.apple.FontBook.plist, ~/Library/Preferences/com.apple.FontRegistry.user.plist and ~/Library/Saved Application State/com.apple.FontBook.savedState, even reinstall Catalina from the Recovery Partition. Nothing had the slightest effect, the missing available fonts are not seen.
Anybody aware of a solution to get Font Book to rebuild the list of available fonts?